Small ice-cream freezers have been of two main types, those having a vertical mixing axis in a vertical cylindrical freezing chamber, and those having a horizontal cylindrical freezing chamber and mixing axis. The mixing axis of the latter type, which is primarily found in commercial installations, introduces a substantial amount of air into the ice-cream producing a product which has a different character from ice-cream produced by methods which do not introduce substantial amounts of air into the mix. Other factors affecting the quality and texture of ice-cream are the freezing temperature, the rate of conduction of heat from the mix during freezing, and the rate of mixing, since these influence the size and character of the ice crystals formed in the mix during freezing. It is widely considered that the best quality ice-cream is produced in the traditional type of ice-cream maker in which the mix is subjected to the action of a hand cranked paddle in a galvanized steel vessel surrounded by a salt-and-ice mixture. The relatively slow, gentle churning action, the stable freezing temperature, and the good conduction of heat from the salt and ice freezing mixture to the ice-cream mix combine to provide ideal freezing conditions. On the other hand, the freezing mixture is messy and corrosive, it requires substantial quantities of ice and salt, and the apparatus is cumbersome, slow and manually operated.
In consequence, various attempts have been made to provide modernized versions of this type of ice-cream maker so as to overcome its disadvantages. One approach has been to provide a vertical freezing vessel with an electrically powered paddle, the entire apparatus being placed inside a domestic freezer. Recent examples of patents relating to such apparatus are U.S. Pat. No. 4,070,957 (Korekawa et al) and No. 4,092,835 (Tanguy et al). Korekawa directs himself to reducing one of the main disadvantages of such apparatus, i.e. it depends on air convection to conduct heat from the freezing chamber and the freezing process is thus undesirably slow, from the point of view both of obtaining acceptably rapid operation and obtaining optimum product quality. Tanguy et al utilize an annular freezing chamber, presumably partly with a view to increasing the heat exchange surface of the freezing chamber and thus speeding up freezing. Tanguy et al also address themselves to another problem of such freezers--that of preventing paddle overload as the product freezes since the apparatus clearly has to be able to operate without direct supervision.
In order to obtain freezing times short enough for ice-cream to be obtained of good quality and without excessive advance notice, the present applicant believes that the apparatus must contain its own refrigeration means, and proposals have in the past been made for such self contained apparatus. A first such proposal is contained in U.S. Pat. No. 3,452,555 (Thurman) which provides an arrangement which seeks to overcome most of the problems discussed above in an apparatus which includes a built-in compressor type refrigeration unit, a freezing vessel surrounded by an evaporator coil, and a motor driven paddle rotatable in a vertical mixing vessel. Unfortunately, the resulting assembly is quite complicated, and a good deal of assembly and disassembly is required to remove and insert the freezing vessel containing the ice-cream mix. In fact, in the preferred embodiment, the evaporator coil is formed in two hinged parts which are clamped together around the freezing vessel, provision being made to unclamp and separate the parts to allow removal of the freezing vessel. Not only does this necessarily make the design of the evaporator coil complex and expensive, but it is also difficult to provide efficient insulation to the parts, and to provide uniform heat transfer to the freezing vessel. Thurman does briefly disclose an alternative embodiment without a split evaporator coil, but does not discuss how the freezing vessel is to be removed from the container.
In U.S. Pat. No. 3,952,538 (Warlick), the inventor overcomes the problem of removing the freezing vessel by giving both it and the evaporator unit a coned shape and rotating the freezing vessel bodily using a bottom drive. This approach, whilst overcoming the problem of removing the container once its contents are formed, severely reduces and renders somewhat erratic the rate of heat transfer between the evaporator and the container, whilst the drive to the vessel must be carefully designed to avoid damage from condensation accumulating or melting into the bottom of the evaporator chamber. The gap between the freezing container and the evaporator must be large enough in practice to prevent the two parts from freezing together as soon as the motor stops. To the best of applicant's knowledge no such integrated ice-cream maker has achieved commercial success on the domestic market.